This invention relates to an optical part or an optical device, and particularly to an optical part or an optical device having an optical material on which minute, generally spherical or aspherical convex or concave arcuate faces are formed and a process of producing the same. Further, the present invention relates to an optical device having an optical path of a great length such as, for example, a higher harmonic wave converting device and a short wavelength laser apparatus employing such optical device.
Conventionally, as a laser resonator, a laser resonator is known of the structure wherein a pair of concave reflecting mirrors are disposed in an opposing relationship to each other on the opposite sides of a laser medium. One of the reflecting mirrors is a total reflecting mirror while the other reflecting mirror is a mirror which passes part of laser light therethrough. In order to miniaturize such a laser system, it is necessary to make the spacing occupied by the reflecting mirrors as small as possible. Further, as miniaturization of systems proceeds, it is necessary to decrease the distance between the reflecting mirrors.
Meanwhile, non-linear optical devices are used which convert a frequency of light making use of a non-linear mutual action of light waves in a substance. The non-linear mutual action may be, for example, production of a second harmonic wave, optical parametric oscillation, production of a difference frequency or the like. Such an optical device is constituted from an optical resonator consisting of two such concave reflecting mirrors as described above, and an optical material in the form of non-linear optical crystal placed between the concave reflecting mirrors. A microlens, which is one of optical parts, is used for an optical communication system or optoelectronics. Since light emerging from a laser or an optical fiber spreads at the angle of 10 to 40 degrees or so, the microlens is used to convert such light into parallel light or further converge such light into a small spot. As one of methods of producing a microlens, there is a method wherein a mask pattern having a necessary circular opening is formed on a glass substrate using the technique of photolithography and ion exchanging is performed through the opening so that the refractive index of a portion of the glass substrate corresponding to the opening is made different from that of the other portion of the glass substrate.
While various optical devices are described so far, if such optical devices can be manufactured by forming a convex arcuate face integrally on an end face of an optical material, then this is very advantageous for miniaturization of an apparatus and so forth. By the way, if it is attempted to form a spherical or aspherical convex arcuate face on a surface of an optical material, one of most popular methods is a method by polishing. Accordingly, it is possible to work only one or several works, and consequently, optical devices cannot be manufactured in a mass at a low cost. Further, where such polishing is involved, there is a problem that, from a physical restriction, a plurality of convex arcuate faces cannot be formed in the proximity of each other on a surface of an optical material. A Fresnel lens is known as an example of an aggregate of small lenses. In this instance, since an electron beam picture drawing apparatus is employed for production of a Fresnel lens, the equipment is very expensive and the mass productivity is low. Further, while it is possible to manufacture, by molding, a microlens on which a plurality of convex arcuate faces are provided, the material of the microlens is limited to such material that is suitable for molding. Accordingly, there is a problem that it is impossible to manufacture a microlens using such a material as a single crystal material or a high melting point amorphous material.
By the way, since generally a laser is limited in output wavelength, there is a method wherein laser light is converted into coherent light of another wavelength making use of a non-linear optical phenomenon in order to obtain laser light of a shorter wavelength. As a representative example of the same, there is a method wherein laser light is converted into a higher harmonic wave by means of a higher harmonic wave generator (SHG). This depends upon the fact that, if laser light of a frequency .omega..sub.o is introduced into crystal of a non-linear medium, then light of the frequency 2.omega..sub.o is outputted from the crystal. Such higher harmonic conversion has been attempted in accordance with various methods, and it is known that a higher harmonic output thus converted increases in proportion to an input power to the second power or an internal power to the second power of a resonator and a length to the second power of the non-linear medium (however, at an optimum focus, the higher harmonic wave output increases in proportion to a length of the non-linear medium). Accordingly, in order to increase the higher harmonic wave output, it is necessary to, increase the internal power or the length of the non-linear medium.
As a method for higher harmonic conversion which is conventionally employed, there is a method of direct conversion wherein non-linear crystal is cut into a piece, and low reflection coatings are applied to the polished opposite end faces of the non-linear crystal piece, and then exciting light is irradiated upon the non-linear crystal piece. With the present method, direct conversion making use of a semiconductor laser is possible, but since generally it is difficult to obtain a long non-linear crystal piece, there is a problem that the available higher harmonic wave output is very low. Further, there is another method wherein a non-linear medium is disposed in a laser resonator. With the present method, since the internal power of the resonator can be made equal to 100 times or so the intensity of incidence light readily, a comparatively high higher harmonic output can be extracted. However, there are problems that the system is complicated and that a separate modulator is required because direct modulation by the semiconductor laser is not performed. Further, as a method which efficiently raises the incidence power and artificially satisfies a phase matching requirement, there is a method wherein exciting light is enclosed by means of a waveguide structure to effect higher harmonic conversion. The method, however, has problems that technical difficulty is involved in production of the waveguide and besides that it is very difficult to introduce exciting light into the waveguide and efficiently extract a higher harmonic wave from the waveguide. Also, there is a problem that, since it is difficult, from a restriction in manufacture, to obtain a symmetrical structure with respect to an optical axis and consequently it is actually difficult to extract a higher harmonic wave of a single mode, it is limited in application.
As described so far, with the methods proposed till now, the output is too low, and if the case is considered wherein an optical device is used as a light source, for example, for an optical disk, then the situation is such that it is difficult to provide a light source device which has a higher harmonic wave output of 1 mW or so and can effect simple modulation.
By the way, a solid-state laser which is longitudinally excited by a laser diode can present a high gain due to a wide spectrum and pumping having a spatial characteristic. Various laser devices of the high output type (including multi-facet pumping devices, LD multiplexing devices and fiber bundle pumping devices) have been developed till now. However, in order to scale up the longitudinal excitation to increase the output power, generally it is necessary to make a cavity large so that the laser device may stand a higher basic mode. Further, if the longitudinal excitation is scaled up, then the gain is decreased due to double refraction or aberration which is caused by heat.
On the other hand, as a method for scaling up the longitudinal excitation while keeping the cavity compact, there is a method wherein, making use of a thermal lens effect which is caused by exciting light introduced into a solid-state laser medium, a pair of flat mirrors are disposed on the laser medium so that multi-array excitation may take place (Oka, M. et at. CLEO '91, p.40). However, this method has problems that, since it makes use of a thermal lens effect, the threshold level in laser oscillation is high and the oscillation efficiency is low.